The oldest vaccine, as everyone knows, is the smallpox vaccine. 1 Another old vaccine (though not in the same class as smallpox vaccine) is the tuberculosis vaccine, Bacillus Calmette-Guérin (BCG), developed in 1921.

There’s all kinds of interesting stuff about BCG. It’s a live vaccine, meaning that the vaccine is live Mycobacterium bovis (the bovine tuberculosis bacterium). Albert Calmette and his assistant/colleague Camille Guérin, at the Institut Pasteur, repeatedly passaged a virulent isolate of M. bovis on a particular type of medium, noticed that some colonies looked different, and determined that these were less virulent for lab animals; after further passages, they announced that the variant was “inoffensif” in humans,2 and by the late 1920s BCG was being used around the world as a vaccine against tuberculosis. Overall, it’s probably around 50% effective as a vaccine, with geographical location accounting for most of the variability. 3 That’s very low as vaccine efficacy goes, and it means that vaccination is really only useful where there’s lots of disease, not so much where there’s a moderate incidence; which pretty much matches how BCG has been used worldwide. On the other hand, the vaccine seems to be pretty innocuous, with a low complication rate.

This much, I was more or less aware of,5 but there’s much more to the story than that, as I’ve recently learned. 4,6 One fascinating question is, exactly what is the vaccine? This is an old vaccine, it was never homogenous (that is, it was basically a crude culture consisting of a wide range of minor variants around a central genome), and it’s been passaged independently all over the world for 85 years. There’s no such thing as “the” BCG any more; there are dozens of different BCGs, some of which can be traced directly back to the original Pasteur stock, others of which have bounced around and been sub-cultured from sub-cultures. There are at least 6 widely-used vaccine strains of BCG,7 not to mention older strains, trial strains, and so on; and most of them behave differently in more or less subtle ways. (See the genealogy of some of the more prominent strains, to the right.) Even the “same” strains have clearly changed over time; for example, somewhere between 1926 and 1934, the Pasteur strain of BCG became drastically less virulent:8

Watson also stated that the strains he had received from Calmette between 1924 and 1927 were potentially virulent for animals, whereas the BCG strains he received and tested between 1929 and 1932 were no longer virulent.9

We don’t have the original BCG strain any more, let alone the virulent M. bovis isolate from which it was derived, but some of the history can be inferred from genome analysis. Unsurprisingly, the different sub-strains have slight variants in their DNA sequences:

These results are best understood from an evolutionary perspective and indicate that BCG has continued to change with in-vitro passage. All BCG strains are lacking deletion region 1, a genetic deletion that may correspond with the altered morphotype observed by Calmette and Guérin. Subsequently, strains obtained before 1926 and maintained on different continents have the 2-IS6110/mpt64+ genotype, which was likely the genetic composition of early BCG. … With such documented genetic change it would be surprising if there has not been phenotypic change over 1173 passages in-vitro, a notion supported in numerous reports describing ongoing attenuation after 1921. … In conclusion, we have demonstrated that through DNA fingerprinting, it is possible to verify the micro-evolution of attenuated Mycobacterium bovis over about one thousand passages. 4

So: We have an old, widely-used vaccine strain, that has a long history of variation. (This is similar, by the way, to smallpox vaccine — vaccinia virus was also widely distributed and showed extensive variation in its characteristics.) The BCG strains used today have all been more or less selected — either deliberately, or empirically — to be safe and effective vaccines; they’ve also been selected for a wide range of other factors (stability, ease of processing, rapid growth, and so on), some of which we know about, some of which we don’t.

Could strain variability account for the wild variation in tested vaccine efficacy? Well. historically, perhaps not — different strains of BCG haven’t correlated with the differences in efficacy that have been seen, most of which (as I said) follow geography.

But I’ll leave you with this: It seems that the BCG vaccines used today are generally less effective than they were back in the 1920s.10 Could that be linked to some of these genomic variations?

In early clinical studies, the live tuberculosis vaccine Mycobacterium bovis BCG exhibited 80% protective efficacy against pulmonary tuberculosis (TB). Although BCG still exhibits reliable protection against TB meningitis and miliary TB in early childhood it has become less reliable in protecting against pulmonary TB.11

Even if we take into account the mysterious shift from cowpox to vaccinia virus as the vaccine strain[↩]

Tuberculosis can involve almost every system of the body and it is essential that doctor belonging to all specialties particularly in developing world should have
skill to diagnose and treat
this common condition
in the most appropriate manner